If I were you I would just pop a really friendly note through the door. As you say, they obviously care for the bun/buns so you don't want to upset them unnecessarily. Either that, or maybe pop round and say you've just seen the bunny through the fence, and you love bunnies yourself so you would love to meet it. Although that one requires you to be a lot braver with face to face contact! Either way, i wouldn't presume they are doing anything wrong until you know any better, as you say they could have provided lots of hay etc and care for bun/buns.